Plasma Enhanced Chemical Vapor Deposition (PECVD) methods of coating external surfaces of a workpiece within a vacuum chamber are well known. The coating of internal surfaces of hollow workpieces using PECVD technique is less common but has been described in U.S. Pat. No. 7,300,684 to Boardman et al., which utilizes a high deposition rate PECVD technique. The Boardman et al. patent is incorporated herein by reference. The method involves using the pipe itself as a vacuum chamber, coupling the gas supply to one opening and the vacuum pump to another, and employing a voltage biasing system connected with the negative terminal attached to the pipe and with return anode(s) located at the ends of the pipe. Hydrocarbon precursors can be introduced and the voltage biasing system is used to generate a high density hollow cathode plasma and attract hydrocarbon ions to the surface to from a diamond-like carbon (DLC) film.
In many cases, it is necessary to coat both the internal and external surfaces of a hollow workpiece. Previously, this required the coating of each surface separately using different processing techniques. The method described in U.S. Pat. No. 6,129,856 to Jung et al. describes a method of coating an internal surface which may result in some unintentional coating of the external surface of the tube, since the entire part is contained within a vacuum chamber. While the Jung et al. method has advantages as compared to other approaches, the thickness of the external coating may be non-uniform and may be less than the thickness of the internal coating, as the gas flow is directed through the inside of the hollow body so that most of the gas will be consumed within the tube. Also, the high ion density hollow cathode plasma rapidly decays after exiting the hollow cathode source, which is in proximity to the internal coating.